Its probably worth it to get one of the slims with more storage. Even though the games you're looking at (older FFs and other older games) aren't going to be very big relatively speaking, storage can still fill up quickly and you never know when you want to play something newer that requires more storage space. In the long run you'll thank yourself for having the additional space!
PSN is a pretty great service (outside of the annoying down times due to maintenance or hacker attempts) , if you sign up for Playstation Plus ($50 for a year membership) you'll have access to (at least) 2 free games every month as well. Officially its 2 free Vita games, 2 free PS3 games, and 2 free PS4 games every month, but a lot of times one of the games on Vita will also be playable on PS3/PS4 so it will be free there as well. The free games tend to be smaller, indie style games but for me that's fine as I like indie games (and some of them lately have been outstanding). Thats in addition to all the sales they run periodically, some of which can be pretty substantial on PSone/PS2 classics.